The invention pertains to a grinding machine for grinding work pieces between centers and for centerless grinding, as well as to a grinding method for grinding a work piece between centers.
The invention pertains, in particular, to a grinding machine that makes it possible to combine the advantages of grinding between centers with those of centerless grinding.
In the well-known traditional method of grinding between centers, the main advantage can be seen in that all ground diameters have the same well-defined center on the work piece axis that lies between the centers. The disadvantage of this method is the higher costs caused by the need for centers. The work piece frequently needs to be processed in steps because a driving device initially needs to be attached in order to turn the work piece.
In centerless cylindrical grinding, in contrast, the work piece is merely placed on a rest between a grinding wheel and a regulating wheel. The rotation of the work piece is realized with the drive of the regulating wheel, and the grinding process is carried out with the driven grinding wheel. Due to this constructive design, there is no need for centers. The advantage of this method can be seen in that it is faster and more economical, particularly for large batch sizes. However, its disadvantages are the usually quite high tool costs and the fact that shafts are, under certain circumstances, no longer concentric to centers or bores, etc.
For example, the commercially available machine “Kronos L dual” by Mikrosa utilizes a technique that is also referred to as dual grinding. Dual grinding is a combination of the above-described methods. The shafts or parts are initially ground between centers. In this case, the work piece is driven by a work piece spindle and a work piece driver. The centricity therefore is ensured in this function “grinding between centers”. Subsequently, the work piece is released from the centers and the centerless grinding process takes place in such a way that the work piece is driven by the regulating wheel and supported on each seat in order to achieve a stable behavior during the centerless grinding. This processing sequence is intended to ensure that the work piece center is not lost and the concentricity relative to the center is preserved. This method is particularly suitable for manufacturing long, narrow shafts because the errors caused by deflection can be better managed during the subsequent centerless grinding. In this case, it is disadvantageous that a work piece driver is still required and that the change-over between operating modes is relatively complicated.
U.S. Pat. No. 3,903,655 discloses a grinding machine with a “resilient” regulating wheel. This grinding machine features a drive for a grinding wheel and a drive for a regulating wheel, wherein the drive of the grinding wheel is apparently designed for a relatively high speed. A work piece is supported between the grinding wheel and the regulating wheel on a work piece axis, namely between centers or other suitable holding means. The driving axes of the grinding wheel and the regulating wheel and the work piece axis are aligned parallel. The grinding wheel, the regulating wheel and the work piece are mounted on separate carriages such that it is possible to adjust the distance between the work piece and the grinding wheel, as well as the distance between the work piece and the regulating wheel. The drive of the regulating wheel serves for driving the work piece by means of frictional forces. For this purpose, the regulating wheel is manufactured of a relatively hard yet “resilient” material. This means that the work piece rotates in a direction and with a speed that are defined by the regulating wheel. In this case, however, the regulating wheel of the grinding machine rotates with a relatively slow speed in comparison with the grinding wheel. This grinding machine is intended for the manufacture of precision machine components.
Although the grinding machine according to U.S. Pat. No. 3,903,655 utilizes certain elements that are also used in centerless grinding (e.g. driving the work piece via a regulating wheel), this machine is not designed for a flexibility that makes it possible to carry out “grinding between centers” as well as “centerless grinding”, and the carriages for the grinding wheel, the regulating wheel and the work piece are all designed for realizing a precise and defined mechanical positioning. However, it is also mentioned that, in principle, not all three carriages need to be provided. The disclosure provides no detailed references as to the generation of a sufficient frictional force between the regulating wheel and the work piece such that it can be ensured that the work piece has and also maintains the required rotational speed during processing. The information provided with respect to the design and the function of the regulating wheel of a relatively hard but “resilient” material is merely of a general nature.